1. Field of the Invention
The present invention relates to semiconductor devices and methods of manufacturing thereof, and more particularly to a semiconductor resistor structure optimized for tolerance and high current and a method of fabrication thereof. More specifically, the present invention provides a high tolerance Temperature Coefficient of Resistance (TCR) balanced high current resistor for RF CMOS and RF SiGe BiCMOS applications and a computer aided design kit for designing the same.
2. Background of the Invention
Optimization of passive elements for tolerance and high current is valuable for RF technologies. In RF circuit applications, precision resistors are needed for I/O circuitry implementing both radio frequency (RF) CMOS an RF SiGe technology. High tolerance resistors are important for accurate prediction of models and statistical control. Moreover, in RF devices and circuits, high tolerance resistors are needed that have good linearity; a low temperature coefficient of resistance (TCR) which is the normalized first derivative of resistance and temperature, and provides an adequate means to measure the performance of a resistor; a high quality factor (Q); and are suitable for high current applications.
In high current RF applications, it is desirable that resistors maintain their structural integrity at high currents. In current multiple inter-level dielectric film stack structures, there exist materials with potentially different thermal and mechanical properties which can influence the temperature distribution within the resistor element and also the mechanical stress and strain in metal and insulation regions. Conventional metal resistor structures subjected to high currents above a critical current-to-failure point, can result in metal blistering, extrusion, and melting of the metal resistor regions. Additionally, subjecting a conventional resistor to high current may result in a thermal gradient in the surrounding insulator that may exceed the yield stress and result in insulator cracking. The above phenomena both reduce the integrity of the dielectric and semiconductor chips when subjected to high current.
Further, in RF CMOS, or RF SiGe, the usage of resistors in series with RF MOSFETs for resistor ballasting in source, drain, and gate regions are valuable for ESD protection. For an RF MOSFET, series resistance is important to minimize for RF performance. Hence, having a low resistance in the source and the drain are important for good RF characteristics. Source and drain resistance are lowered using salicide regions on the source and drain diffusion regions, but salicide near the gate impacts the ESD robustness of the device. For an RF MOSFET device, it is key to provide ballasting effects as well as low resistance. Adding extra resistor elements increase the loading capacitance on the circuit and impacts area. Hence, finding a means to provide low resistance for RF functionality but ballasting for ESD robustness is key to providing a good RF MOSFET.
It is also well known that current drive in devices at high current is not uniform, largely due to non-uniform temperature distribution in such devices when driven at high currents. Thus, to provide uniformity of current drive, a device which has a more uniform current distribution as a function of device dimensions is an advantage.
Moreover, for RF bipolar and SiGe transistors, a means for establishing uniform current in a transistor to maximize its high current capability is key for power amplifier applications, ESD networks and other applications. Current uniformity can lead to an improved net performance by avoiding increasing a structure size to provide an equivalent drive strength. Additionally, using resistor ballasting in a base region can lead to uniformity of input current. Additionally, using a resistor ballasting in an emitter structure can provide both thermal and electrical stability in a circuit. Additionally, it is important that the element does not structurally fail due to high currents. For differential circuits, it is important that good matching characteristics are present in the physical elements.
It would therefore be highly desirable to provide a semiconductor resistor structure and method of fabrication that is customized to achieve a desired (optimized) TCR, and preferably, a low net Temperature Coefficient of Resistance (TCR) value at high currents and in a joule-heating regime of operation. To this end, it would be desirable to provide a semiconductor resistor element structure and method of fabrication for power amplifiers, and ESD applications that provides a tunable Temperature Coefficient of Resistance for circuit linearity.
It would furthermore be highly desirable to provide a semiconductor resistor element structure and method of fabrication, wherein the resistor element is capable of carrying high currents without failure, and is designed to exhibit internal self-resistor ballasting to maintain a uniform current density and thermal gradient for uniform current distribution and minimization of thermal stress.
It would moreover be highly desirable to provide a semiconductor RF MOSFET device implementing a high resistance element that is physically small, provides a high Q factor, and renders the device electrically and thermally stable at high temperatures and high currents.
ESD protection circuits for input nodes must also support quality dc, ac, and RF model capability in order to co-design ESD circuits for analog and RF circuits. With the growth of the high-speed data rate transmission, optical interconnect, wireless and wired marketplaces, the breadth of applications and requirements is broad. Each type of application space has a wide range of power supply conditions, number of independent power domains, and circuit performance objectives. As a result, an ESD design system which has dc and RF characterized models, design flexibility, automation, ESD characterization, and satisfies digital, analog and RF circuits is required to design and co-synthesize ESD needs of mixed signal RF technology.
The ability to design a resistor element so that co-synthesis of the ESD and the functional RF needs to insure integrity of the resistor element is critical in future technologies.
Much effort has been expended by industry to protect electronic devices from ESD damage. Traditionally, ESD designs are custom designed using graphical systems. ESD ground rules and structures are typically built into the designs requiring a custom layout. This has lead to custom design for digital products such as DRAMs, SRAMs, microprocessors, ASIC development and foundry technologies. This design practice does not allow for the flexibility needed for RF applications. A difficulty in the design of RF ESD solutions is that traditionally, specific designs are fixed in size in order to achieve verifiable ESD results for a technology. The difficulty with analog and RF technology is that a wide range of circuit applications exists where one ESP size structure is not suitable due to loading of the circuit. A second issue is that the co-synthesis of the circuit and the circuit must be done to properly evaluate the RF performance objectives of a resistor element. RF characterization of the resistor or network that is flexible with the device size is important for the evaluation of the tradeoffs of RF performance and ESD. A third issue for RF mixed signal designs, there are analog and digital circuits.
In this environment, the verification and checking is necessary to evaluate ESD robustness of the resistor element and the ESD robustness of the semiconductor chip. The verification of the existence of the ballast resistor elements, the pads, the ESD input circuit, the ESD power clamp circuit, ESD rail-to-rail circuits, interconnects between the input pad and the ESD circuits, interconnects between power pads and the ESD power rails, the interconnects between two power rails for rail-to-rail ESD networks, the verification of ESD rail-to-rail type designs between functional blocks, verification of type of ESD networks on analog, digital and RF circuits, verification of the correct ESD network for a given chip circuit, verification of the critical size of the resistor, and the interconnects, verification of the size and adequacy of the ESD network are all important to provide ESD protection of RF BiCMOS, RF BiCMOS Silicon Germanium and RF CMOS applications.
It would thus be further highly desirable to provide a computer aided design tool with the ability to provide customization and personalization of the internal ballasting (both lateral and vertical), variable TCR, TCR matching, high current robustness, electrothermal optimization and ESD robustness.
It would additionally be desirable to provide a computer aided design tool with graphical and schematic features hierarchical parameterized cell for a resistor element with the ability to provide customization, personalization and tunability of TCR, TCR matching, and high current robustness and ESD robustness.
It would further be highly desirable to provide a computer aided design tool with graphical and schematic features hierarchical parameterized cell which allows graphical or schematic optimization and autogeneration of the resistor element.